Water Demand Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Water systems are designed for more than an average day. Utilities need to account for normal per-capita demand, an additional allowance, maximum-day conditions, and short peak flows. This calculator turns those planning assumptions into average demand, maximum daily demand, and a peak design flow.

What this calculator does

The Water Demand Calculator estimates community water demand from population and daily per-capita requirement. It applies an additional allowance percentage to form average daily demand, multiplies by a maximum-day factor, and converts the peak factor into liters per second. The headline result is average demand in liters per day.

How to use it

Enter population size and per-capita daily water requirement, then choose the additional allowance, maximum-day factor, and peak-hour factor. The per-capita field can be entered in liters or US gallons and is converted internally. Use factors that match the design standard or local utility practice you are analyzing rather than assuming the defaults apply everywhere.

How the calculation works

Average daily demand = population × per-capita requirement × (1 + allowance). Maximum daily demand = average × maximum-day factor. Peak design flow in the current calculator = average daily demand × peak factor ÷ 86,400 seconds/day, producing L/s. The peak calculation therefore scales the average day into an instantaneous design-flow equivalent.

Example

For 100,000 people using 225 L/person/day with a 10% allowance, average demand is 24,750,000 L/day. A 1.4 maximum-day factor gives 34,650,000 L/day. With a 2.3 peak factor, the calculator produces a peak design flow of about 658.85 L/s.

How to interpret the result

Use the average result for overall supply scale, the maximum-day figure for high-demand daily conditions, and the L/s figure for peak-flow sizing discussions. The difference between them is intentional. Larger peaking factors increase infrastructure design flow without changing the underlying average per-person water-use assumption.

Limitations and notes

Real water-demand design can include non-revenue water, industrial and commercial demand, firefighting, seasonal population, climate, conservation policy, hourly demand curves, storage, pressure zones, and local code requirements. This calculator uses a compact factor method. Do not substitute its defaults for the factors required by a specific municipality or engineering standard.